A Power Split Transmission (PST) (also known as an Electric Variable Transmission (EVT)) is one type of transmission used for Hybrid Electric Vehicles (HEV). Power split transmissions utilize two electric motors (“E-motors”) and require completely new transmission designs over conventional transmissions and existing HEV transmissions with a single E-motor. Power split transmissions utilize epicyclic differential gearing (i.e., planetary gear sets) in a way that a part of the power of the internal combustion engine is transmitted through a mechanical path to the output, and another part is transmitted on an electrical path. The electrical path, includes the two E-motors, one of them operating as a motor, and the other as a generator. By varying the speeds of the E-motors, variable transmission ratios are obtained.
Typically, the PST allows no carry over of parts from conventional transmissions resulting in large development costs. However, HEVs with PSTs provide improved fuel economy over traditional series and parallel hybrids because the E-motors not only boost or regenerate during braking, but also drive in variable ratios. Those operations are done in EVT modes. Conventional geared transmissions have a certain number (e.g., four to seven) of gears with fixed ratios. Variable ratios in the EVT modes allow the engine to operate near the best efficient load point. In existing HEV PSTs, high-power E-motors and power electronics are required in order to cover full load engine operation. Existing power split transmissions do not include very low power split ratios, four well-distributed gears, and a front E-motor design.